Removal of (Abstract-)TestFixture

As of NUnit's version 2.5 it's not needed anymore to mark a class as TestFixture, every class that contains a test is considered a test fixture.

I removed the logic relying on something to be a marked as [TestFixture]/[AbstractTestFixture] to distinguish if a class is a Testeroids class. Now it simply checks for classes implementing IContextSpecifcation containing tests.

Removed compile time validations for AbstractTestFixtures as they are not needed anymore.

Changed [PublicAPI] to [UsedImplicitly] in TypeInvestigationService.